The invention relates generally to power-driven conveyors and more particularly to direct-drive spiral conveyors and methods for their operation.
In drum-driven spiral conveyors a conveyor belt is driven in a helical path winding up or down the periphery of a rotating cylindrical drive drum, or tower. In so-called low-friction spiral conveyors, the periphery of the drive drum frictionally engages the inside edge of the belt to drive it along the helical path. The drum rotates at a greater angular speed than the belt. In other words the drive drum is overdriven, and the belt slips relative to the drum. The constant slipping of the belt causes vibrations in the belt that can change the orientations of conveyed products. The drive drum of a direct-drive, or positive-drive, spiral conveyor has vertical drive bars spaced apart around the drum's periphery. The drive bars positively engage the inside edges of the conveyor belt and drive it along its helical path without slip and with less belt vibration.
The tension in a long spiral conveyor belt can be quite high. One way tension in a direct-drive spiral belt is reduced is with a drive drum having a greater diameter at the entrance of the belt onto the drum than along the remainder of its helical path about the drum. The large-diameter portion of the drum causes the belt to stretch upon entry. Then, as the belt makes its way to the smaller-diameter portion of the drum, the belt, like a rubber band, relaxes somewhat, and its tension decreases. Putting a large-diameter portion on a drive drum requires that all the drive bars be built out radially along a portion of their lengths near the entry to the drum.